1. Field of the Invention
Aspects of the present invention relate to a polymer membrane, a method of preparing the same, and a fuel cell using the polymer membrane, and more particularly, to a polymer membrane capable of effectively decreasing crossover in a fuel cell by infiltrating pores of a porous polymer matrix with an acryl-based polymer, a method of preparing the same, and a fuel cell using the polymer membrane.
2. Description of the Related Art
Fuel cells generate electrochemical energy by reacting fuel and oxygen and are used as electric power sources for industrial use, domestic use, vehicles, and small electric/electronic applications, such as portable devices, etc.
Fuel cells can be categorized into polymer electrolyte membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), etc., according to the electrolyte that is used. These fuel cells have different operating temperatures and include different constituting material, according to the electrolyte that is used.
According to the method of supplying fuel to an anode, fuel cells can be categorized into external modification type fuel cells, in which fuel is supplied to an anode after being converted into hydrogen-rich gas by a fuel modifier, and direct fuel supply type or internal modification type fuel cells, in which gaseous or liquid fuel is directly supplied to an anode.
Direct methanol fuel cells (DMFCs) are a kind of direct fuel supply type fuel cells. In general, a DMFC includes an aqueous methanol solution as fuel and a proton conductive polymer electrolyte membrane as an electrolyte. Accordingly, DMFCs also belong to the group of PEMFCs.
PEMFCs can exhibit high power output density even when they are small and lightweight. Furthermore, an energy generating system can be simply constructed using a PEMFC.
In general, a basic structure of a PEMFC includes an anode (fuel electrode), a cathode (oxidant electrode), and a polymer electrolyte membrane interposed between the anode and the cathode. The anode of the PEMFC includes a catalyst layer that promotes oxidation of fuel, and the cathode of the PEMFC includes a catalyst layer that promotes reduction of an oxidant.
Fuel that is supplied to the anode of the PEMFC can be hydrogen, a hydrogen-containing gas, a mixed gas of methanol and water, an aqueous methanol solution, etc. An oxidant that is supplied to the cathode of the PEMFC can be oxygen, an oxygen-containing gas, or air.
In the anode of the PEMFC, fuel is oxidized and thus generates protons and electrons. The generated protons are transported to the cathode through the electrolyte membrane, and the generated electrons are transported to an external circuit (load) through a conductive line (or current collector). In the cathode of the PEMFC, protons transported through the electrolyte membrane, electrons transported from the external circuit through the conductive line (or current collector), and oxygen are combined together to generate water. Meanwhile, the flow of electrons from the anode to the cathode through the external circuit produces electrical power.
The polymer electrolyte membrane of the PEMFC acts as an ion conductor that allows protons to move from the anode to the cathode and also as a separator that mechanically separates the anode from the cathode. Accordingly, desired properties of the polymer electrolyte membrane include excellent ion conductivity, electrochemical stability, high mechanical strength, thermal stability at operating temperatures, ease of thin film formation, etc.
A polymer electrolyte membrane is typically formed of a polymer electrolyte, such as a perfluoro sulfonated polymer (for example, NAFION (Dupont Inc.)) that has a main chain of fluorinated alkylene and a side chain of fluorinated vinyl ether terminated with a sulfonic acid group. Such a polymer electrolyte membrane exhibits excellent ion conductivity by containing an appropriate amount of water.
However, since the polymer electrolyte membrane has a channel of a large diameter within an ionomer cluster, the crossover rate of fuel (that is, the rate at which fuel crosses through the membrane from the anode to the cathode without producing electricity) is high. Moreover, when the polymer electrolyte membrane is assembled to form a fuel cell, it may be easily bent due to its poor mechanical properties. Accordingly, it is difficult to manufacture a fuel cell using a polymer electrolyte membrane.
In order to address these problems, a method of preparing an organic-inorganic hybrid material by cross-linking a precursor that is an organosilicon compound having a mesogenic group (U.S. Patent Application Publication No. 2005-100772), a method of preparing a polymer electrolyte membrane containing both a fluorinated polymer and a nonfluorinated polymer (U.S. Patent Application Publication No. 2004-0241519), and a method of preparing a polymer electrolyte membrane by swelling a fluorinated ionomer with an organic solvent, removing the used organic solvent, and adding a vinyl monomer, an inhibitor, and various additives to the swollen fluorinated ionomer fluoride (U.S. Pat. No. 4,200,538) have been suggested.
However, all of these conventional techniques described above have not effectively suppressed the crossover phenomenon in a fuel cell.